Re: Switching to udev?

[Demi Marie Obenour <demiobenour@gmail.com>]

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On 7/26/25 06:06, Alyssa Ross wrote:
> Demi Marie Obenour <demiobenour@gmail.com> writes:
> 
>> On 7/23/25 13:34, Demi Marie Obenour wrote:
>> From https://wiki.gentoo.org/wiki/Mdev:
>>
>> - udev is needed to load firmware for AMD Radeon GPUs.
>> - libusb will not see USB devices unless deprecated kernel
>>   configuration options are set.
>>
>> From https://wayland.freedesktop.org/libinput/doc/latest/architecture.html:
>>
>> - Wayland compositors use udev (via libinput) to support
>>   device hotplugging.  This is needed for USB keyboard
>>   and/or mouse support, which is a hard requirement for
>>   desktops that don't have PS/2 connectors (almost all of
>>   them I believe).
>> - libinput relies on udev to determine what kind of device
>>   an input device is.
>>
>> FreeBSD also switched to udev for Mutter.
>> https://bugs.freebsd.org/bugzilla/show_bug.cgi?id=271824#c4
>> states that for Wayland, udev is essentially required.  Even
>> Gentoo uses udev, though systemd is not the default init
>> system.
>>
>> I think avoiding udev is a losing battle in the long run,
>> except possibly for VMs that will never ever have physical
>> hardware attached to them.  That would require configuring
>> VMs with and without physical hardware separately, which
>> I would strongly prefer to avoid.  If the memory
>> consumption or disk space requirements of udev are
>> excessive, I think this would be better addressed as part
>> of a broader debloating effort.
>>
>> libudev-zero does exist, but it doesn't work with PipeWire
>> (https://github.com/illiliti/libudev-zero/issues/26, still
>> unfixed despite being closed) and there are other problems
>> with it as shown by its issue tracker.  Alpine's wiki doesn't
>> recommend it for desktop environments, and while Spectrum VMs
>> don't run full desktop environments they do run desktop
>> software.  Such software requires udev more and more nowadays.
> 
> Broadly it seems like no libudev at all (which was never on the table)
> is being conflated with not using udevd (but using something like
> libudev-zero) here.  To my knowledge, everything you've outlined should
> work with libudev-zero, with the exception of PipeWire as you point out,
> and possibly Radeon firmware loading — I'm very surprised udev would be
> involved at all in that process so wondering if the wiki is very out of
> date or something.  We have USB input working on the host using
> libudev-zero, for example.
> 
> The PipeWire compatibility problems do seem like blockers, but reading
> the PipeWire issue about it[1], I'm not sure they're fundamental.  It
> sounds like PipeWire might be willing to accomodate a fix, the last
> activity on the issue was only a month ago, and it doesn't sound like
> that big of a change is necessary for it to work.  I wouldn't be
> surprised if PipeWire is among the software that has the most complex
> interaction patterns with udev, and given it might be on the verge of
> being fixed, I'd prefer to wait and see a bit longer.  If that means
> that we don't have working audio hotplug or whatever in the meantime,
> that's okay.  I've put the PipeWire issue on the upstream bug tracking
> board[2].  If months go by and no solution is found, I think it would
> make sense to evaluate switching to systemd-udevd then, but I don't want
> to do the switch, and then find a weak later that the only firm reason
> we had for doing it has been solved.  If you know about other software
> that depends on custom udev properties, and isn't on the verge of being
> fixed like PipeWire is, or if I'm wrong about Radeon, we could also take
> that into account now, but so far all we know for sure is that PipeWire
> doesn't work, but might do quite soon.
> 
> [1]: https://gitlab.freedesktop.org/pipewire/pipewire/-/issues/2398#note_2897989
> [2]: https://cryptpad.fr/kanban/#/2/kanban/view/yLtGXWLV6U7X5+Z1ay+oXKZMrSacqQe+51nXZYRh3ck/

I think libudev-zero can work fine in the VMs that have no physical
hardware, especially with flatpaks that can't access udev at all.
It's the host and VMs with physical hardware we don't have, and thus
cannot test, that I am concerned about.  systemd-udevd is much more
likely to have gotten a patch from someone else to fix an issue we
would never run into ourselves, and which might well cost us a user.
-- 
Sincerely,
Demi Marie Obenour (she/her/hers)

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